Microstrip antenna system adaptively steerable in two dimensions

ABSTRACT

An antenna system for adaptive steering in azimuth and elevation. The  sys utilizes a cross-shaped, planar, microstrip array with a central receiving element and with arms of parasitic elements having individual phase shifters which are simultaneously adjusted. In steering a null, the phase shifters are initially adjusted in a &#34;random search&#34; mode and are then adjusted in a &#34;guided random search&#34; mode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to the field of antennas used fordirective radio wave communications. More particularly it applies tomicrostrip antennas having a steerable array with parasitic elements andelectronically controlled scanning.

2. Description of the Prior Art

It is known to utilize an antenna system having a linear microstriparray with a central receiving element and parasitic elements extendedoppositely therefrom to adaptively steer an antenna pattern by phaseshifters individual to the parasitic elements, such a system beingparticularly useful to steer a null toward an undesired microwavesource. However, it is highly desirable to provide an antenna systemhaving the compactness and simplicity of a microstrip array butsteerable in azimuth and elevation in real time.

It is also known to steer such a linear microstrip array by circuitsindividual to the phase shifters where each such circuit has aoscillator providing a reference signal modulating the phase shifter,has a synchronous detector which receives the reference signal and theoutput of a receiver connected to the central elements, and has anintegrator connected to the detector output and driving the phaseshifter to minimize or maximize the receiver output. This manner ofsteering a microstrip array requires many circuit elements and is noteasily adapted to other methods of steering to optimize the arraypattern.

"Random search" and "guided random search" are iterative methodswell-known for optimization when a number of variables must be adjusted.These methods have the advantage of rapid real-time convergence sinceall variables are adjusted simultaneously with the result thatconvergence may occur in less time than in other well-known optimizationmethods such as "steepest descent" where, although fewer completeiterations of all variables are required, the total time required isgreater since at each iteration every variable must be adjustedindividually and the effect measured.

SUMMARY OF THE INVENTION

The present invention is a microwave antenna system having a planar,cross-shaped, microstrip array with a central receiving element andorthogonally related arms consisting of parasitic elements whose phaserelations are controlled by phase shifters individual thereto. The phaseshifters are digitally and simultaneously controlled to steer the arrayin two-dimensions in relation to a radiating source by a random searchmode followed by a directed random search mode.

It is an object of the present invention to provide a micro-stripantenna system steerable in azimuth and in elevation.

Another object is to provide such a system which rapidly steers anantenna pattern in relation to a radiating source.

A further object is to provide such a system particularly effective insteering a null toward such a source.

BRIEF DESCRIPTION OF THE DRAWING

Other objects, advantages, and novel features of the subject inventionwill become apparent from the following detailed description thereofwhen considered with the accompanying drawing in which the FIGURE is aschematic representation of an antenna system embodying the presentinvention and has, at the left side, a perspective representation of amicrostrip array and a beam steered thereby and has, toward the rightside, a block diagram of electronic elements used with the array.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The FIGURE shows an antenna system 10 which embodies the presentinvention and adaptively steers an antenna pattern or beam 11 which, forillustrative convenience, is depicted as a lobe but may be a null to bedirected toward an undesired source, not shown.

System 10 has a planar, cross-shaped microstrip array 20 with a centralelement 21 and eight parasitic elements 22. Elements 21 and 22 are of awell-known rectangular configuration and elements 21 are disposed in apair of orthogonally related arms 23 intersecting at and extending fromelement 21. Elements 21 and 22 are dimensioned and spaced, in a mannerwell-known to those familiar with the art of microstrip array design, sothat array 20 is configured for a predetermined microwave frequency withelements 21 and 22 resonant thereat. The number of elements 21 in eacharm 23 may be increased to narrow beam 11. Array 20 is, typically,arranged so that the longer axis of elements 21 and 22, which definesthe electric or E plane 25 of array 20, extends in azimuth and so thatthe shorter axis of these elements, which defines the magnetic or Hplane 26 of the array, extends in elevation.

System 10 includes any suitable receiver 30 having a microwave frequencyinput 31, which is connected only to central element 21, and having anoutput 32 which provides a signal proportional to the power received bythe central element at the frequency for which array 20 is configured.This signal is provided externally of system 10 for any desired purposeby a connection 33 and is provided to an analog-to-digital converter(A/D) 35 which provides a digital measurement of the microwave powerreceived by element 21.

System 10 has eight varactor phase shifters 40 individually connected toparasitic elements 21 and having individual control inputs 41. System 10has eight digital-to-analog converters 45 individually providing controlsignals to inputs 41. The construction of phase shifters 40 and theirconnections to array 20 and converters 45 are any well-known to the artsuch that each signal from a converter 45 to an input 41 has apredetermined voltage range which corresponds to a predetermined changein the phase angle of the associated one of the parasitic elements 21 inrelation to the others thereof.

System 10 includes a digital steering computer 50 which accepts, throughanalog-to-digital converter 35, power measurements for central element20, and which drives, by connections 55, digital-to-analog converters45. Computer 50 is of any suitable configuration programmable to vary,substantially simultaneously and in accordance with a predeterminediterative method, the signals from converters 45 based upon the measuredpower from central array element 21. In order to obtain rapidconvergence in such a method, the relative phase of every one of theparasitic elements 22 is varied substantially simultaneously at eachiteration in relation to the others thereof and in response to onemeasurement of the power from element 21. This simultaneous variation isobtained by computer 50 and converters 45 simultaneously changingsignals 41.

System 10 utilizes a preferred iterative method to steer beam 11 in twodimensions to obtain a predetermined power output from receiver 30, thisoutput being a minimum when it is desired to steer a null. This methodgenerates repeated variations of the relative phases of all of theparasitic elements, these variations forming corresponding antennapatterns resulting in different power outputs from receiver 30corresponding to the energy received by element 21 and measurable ateach variation by analog-to-digital converter 35. In the preferredmethod, random variations are generated within the range of the signalsto phase shifter control inputs 41, and the one of such variationsresulting in measured power from receiver 30 nearest to the desiredpower output is selected.

Initially in the preferred method, a "random search" is utilized inwhich the distribution of values within the input range to phaseshifters 40 is determined at each variation by a random number generatorso as to be distributed with a uniform density over this entire range.After a predetermined number of iterations, typically twenty, thesevalues corresponding to the random search iteration resulting, if a nullis to be steered, in the lowest power from receiver 30 are selected asthe starting point for a following "guided random search".

In this guided random search, the density during successive iterationsof the values within the input ranges of the phase shifters 40 isprovided by Gaussian random number generator which generates a sequencewith a zero mean and a predetermined standard deviation. At eachiteration, the Gaussian values are output simultaneously to all thephase shifters 40 and a single measurement of the power at receiveroutput 32 is made. If these values reduce this power, they are used as astarting point for the next iteration; otherwise, such values from theprevious iteration are used to continue the guided random search.

During the guided random search the standard deviation mentioned in theprevious paragraph must not be so small that a small minimum isoverlooked, but must not so large that convergence does not occur at asteep minimum. An effective standard deviation for this purpose can bedetermined by trial and error. However, use of the present inventionwith an array, which was like array 20 in having eight cross-configuredparasitic elements and a range for phase shifter 40 of approximately 250degrees from range of 10 volts on control inputs 41, gave satisfactoryconvergence with a standard deviation of 0.15 of the voltage range.

What is claimed is:
 1. An adaptively steerable antenna systemcomprising:a planar microstrip array configured for a predeterminedmicrowave frequency and havinga central element, and a plurality ofparasitic elements disposed in a cross having a pair of orthogonallyrelated arms crossing at the central element; a receiver having an inputconnected only to the central element and providing an output signalproportional to the power received at said frequency by the centralelement; a plurality of individually controllable phase shifting means,which are individual to the parasitic elements, for changing the phaserelation of said parasitic elements at said frequency; and means forcontrolling each of said phase shifting means of said plurality thereofin accordance with a predetermined method to minimize said outputsignal.
 2. The antenna system of claim 1 wherein said method isiterative and at each iteration, based on one measurement of said powerreceived by the central element, the relative phase of substantiallyevery one of said parasitic elements is varied in relation to the othersthereof.
 3. The antenna system of claim 2 wherein said method comprisesa random search followed by a guided random search.
 4. The antennasystem of claim 1 wherein:each of said phase shifting means accepts acontrol input signal having a predetermined range and shifts said phaserelation of the corresponding parasitic element over a rangecorresponding to said predetermined range; and said controllingmeansprovides a plurality of control input signals individually to saidphase shifting means, and simultaneously changes said plurality ofcontrol input signals by a random variation within said predeterminedrange.
 5. A method of steering in two dimensions a beam of a microwaveantenna system to obtain a predetermined power output therefrom, themethod comprising:providing the system witha planar microstrip arrayhaving a central element and a plurality of parasitic elements arrangedin a cross configuration with orthogonally related arms extending fromthe central element, a receiver having an input from the centralelement, a plurality of varactor phase shifters individual to theparasitic elements, and means for controlling each of said phaseshifters to vary within a predetermined range the relative phase of thecorresponding one of the parasitic elements in relation to the othersthereof; generating repeated variations of the relative phases of all ofthe parasitic elements by varying the relative phase of each thereofwith a predetermined distribution within said range; measuring the powerreceived by the central element at each variation of said relativephases; and selecting a one of said variations having such a poweroutput nearest to said predetermined power output.
 6. The method ofclaim 5 wherein said predetermined power is a minimum so that a null ofsaid antenna system is steered thereby.
 7. The method of claim 5wherein, during said repeated variations of said relative phases, saidpredetermined variation is initially random over said entire range andthe selected one of such variations is utilized as a basis for furthersuch repeated variations in which said predetermined variation isGaussian.